The disclosure relates to a method for adjusting an amplitude of a voltage injection in a determination of a rotor position of a rotating, multi-phase electric machine which is fed by means of a PWM-controlled inverter by way of an anisotropy-based method.
Such a method is disclosed for example in the article “Audible Noise Reduction Method in IPMSM Position Sensorless Control based on High-Frequency Current Injection” by Yuki Tauchi et al., which was published in 2014 at the “International Power Electronics Conference” of the IEEE. It is possible by means of the adjustment of the amplitude of the voltage injection to influence the signal-to-noise ratio of the ascertained rotor position. In the case of fast speed changes it is necessary to reduce the filter time constant of the rotor position determination, such that the calculated rotor position follows the real rotor position without too much of the delay. In order that the calculated rotor position is nevertheless not too noisy, the signal-to-noise ratio must be improved by increasing the amplitude of the voltage injection. Accordingly it is possible in the case of a constant speed of the machine to increase the filter time constant, as a result of which the signal-to-noise ratio is improved. It is thus also possible to decrease the amplitude of the voltage injection. In the article mentioned, for adjusting the amplitude of the voltage injection, the deviation between a target speed and an actual speed of the electric machine is determined, and the amplitude of the high-frequency voltage injection is adjusted in dependence on said deviation.
In the case of sensorless control, an attempt is made to ascertain the rotor position of an electric machine without using a position sensor. So called anisotropy-based methods can be used here, which determine the rotor position via the magnetic anisotropy of the rotor. In the case of these anisotropy-based methods, in addition to the injection of a voltage to be set for the electric machine, a high-frequency voltage injection also takes place, which results in a high-frequency current change in the phase currents of the electric machine. This current change contains information relating to the rotor position and consequently it is possible to determine the rotor position from the current change via a suitable model approach.